Diffusion of water in silica: Influence of moderate stresses

Here we study the diffusive transfer of water into silica glass in the presence of externally applied stresses, and stresses caused by water induced swelling of the glass. By considering the simultaneous action of water penetration into the surface of silica glass and the development of swelling stresses in the water‐penetrated zone, several experimental findings previously published in the literature can be interpreted quantitatively. These include an apparent decrease of the diffusivity with time, an increase of water solubility in the surface region under compressive loading, the opposite effect under tensile loading and a reversal of these two effects deeper within the glass. These expectations are fully met in published experiments carried out to date.     

Surface Crystallization and Water Diffusion of Silica Glass Fibers: Causes of Mechanical Strength Degradation

Pristine silica glass fiber is well‐known to become mechanically weaker when heat‐treated in air but the cause of such weakening is not presently known. The time dependence of mechanical degradation of various silica glass fibers containing varying impurity contents were studied in the range from 500°C to 1000°C. Two possible sources of strength degradation were considered: surface crystallization and water diffusion. Surface crystallization kinetics of silica glass fibers were investigated in a wide temperature range, including nanoscale surface nucleation at low temperatures via scanning electron microscopy. From the comparison of the strength degradation, surface crystallization, and water diffusion data in literature, it was concluded that surface crystallization may be responsible for the mechanical weakening observed in silica glass fiber surface during heat‐treatment at temperatures above ~800°C, whereas water diffusion into the glass surface may be responsible for the strength degradation at lower temperatures.     

Scanning Tunneling Microscopy of Optical Fiber Corrosion: Surface Roughness Contribution to Zero-Stress Aging

Strength, fatigue resistance, and zero-stress aging behavior control the long-term mechanical reliability of optical fibers. Zero-stress aging refers to the loss of strength of high-strength glass fibers after exposure to some corrosive environments in the absence of stress. Understanding the effect of the chemical environment under zero stress on the subsequent fracture strength of optical fibers is important because optical fibers in service will probably encounter water and other chemical species while exposed to zero- or low-stress conditions. In this work, the strength of fibers aged under zero-stress conditions at 80°C in deionized water has been measured. Scanning tunneling microscopy was also used to measure the roughening of the fibers from corrosion at intervals during the aging. The product of the median inert strength of fibers aged for various times and the square root of the roughness depth of fibers was constant within experimental error. The results show that surface roughening contributes to zero-stress aging in silica fibers.     

Modeling Slow Crack Growth Behavior of Glass Strengthened by a Subcritical Tensile Stress Using Surface Stress Relaxation

Glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. It has been observed by several authors that when an oxide glass with a large crack is held under a subcritical stress intensity (where no slow crack growth occurs) in room‐temperature water vapor or liquid water, upon reloading to a higher stress intensity, a finite restart time is observed prior to measurable crack extension. This phenomenon of apparent strengthening, or crack arrest, has been attributed to concepts such as corrosive dissolution of the crack tip, crack tip blunting, or water diffusion, and subsequent swelling of the material around the crack tip. Recently, a newly observed surface stress relaxation process that is aided by molecular water diffusion was used to improve the mechanical strength of glass fibers and to explain the subsurface compressive stress peak observed in ion‐exchange strengthened glasses. The same process is employed here to explain these delayed slow crack growth data. A simple mathematical model has been developed utilizing water‐assisted surface stress relaxation and fracture mechanics. Predictions of restart times using the model agreed well with published experimental data, indicating that surface stress relaxation is responsible for the anomalous delayed slow crack growth behavior.     

Effect of Surface Energy on the Mechanical Strength of a High-Silica Glass

The mechanical strength of high-silica glass was measured in various organic liquids and in water. The extent of the surface-energy reduction of glass was also estimated from the swelling or the expansion of porous glass, immersed in liquids, by measuring the stress in the core of a partially leached glass. The stress, normalized to the value in water, showed a good correlation with the heat of immersion, which is a measure of the surface-energy reduction. Mechanical strength and the stress due to swelling showed a reasonable correlation, suggesting that the strength variation in various liquids is due to the surface-energy variation and the accompanying force which caused the swelling.     

Effects of alkaline and silane treatments on the water‐resistance properties of wood‐fiber‐reinforced recycled plastic composites

The water‐resistance properties of wood‐fiber‐reinforced recycled plastic composites (WRPCs) prepared from postconsumer high‐density polyethylene (HDPE) and wood fibers from saw mills were studied. Three methods consisting of an alkaline method (AM), a silane method (SM), and a combination of the alkaline and silane methods (ASM) were used to modify the wood fibers. The effects of fiber/matrix mix ratio and surface treatment on the moisture content, thickness swelling, and flexural strength change of the WRPCs, before and after immersion in 60°C water for 8 weeks, were studied and analyzed. The flexural fractured surfaces of the WRPCs before and after immersion in hot water were examined, and the fracture mechanism of the WRPCs was discussed. The results showed that the different surface treatments of the wood fibers had significant effects on the moisture content, thickness swelling, and flexural strength of the WRPCs after a long immersion time in hot water. For WRPCs treated by ASM, the moisture content was the lowest, the thickness swelling was at a minimum, and the flexural strength was the highest. Higher water absorption of composites with fiber treated by the AM or SM methods, as compared to those treated by ASM, could be attributed to the incomplete adhesion and wettability between the wood fibers and the polymer matrix, which may have caused more gaps and flaws at the interface. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.     

Flexural properties of S‐2 glass and TR30S carbon fiber‐reinforced epoxy hybrid composites

A study on the flexural properties of hybrid composites reinforced by S‐2 glass and TR30S carbon fibers is presented in this article. Test specimens were made by the hand lay‐up process in an intraply configuration with varying numbers of glass/epoxy laminas substituted for carbon/epoxy laminas. These specimens were then tested in the three point bend configuration in accordance with ASTM D790‐07 at a span to depth ratio of 32. The failed specimens were examined under an optical microscope, and the results show that the dominant failure mode is at the compressive side. The flexural behavior was also simulated by finite element analysis (FEA). Based on the FEA results, the flexural modulus and flexural strength were calculated. Good agreement is found between the experiments and FEA. It is shown that flexural modulus decreases with increasing percentage of S‐2 glass fibers, positive hybrid effects exist by substituting carbon fibers for glass fibers, and applying a thin layer of S‐2 glass fiber‐reinforced polymer on the compressive surface yields the highest flexural strength. The modeling approach presented will pave a way to the effective design of hybrid composites. POLYM. COMPOS., © 2012 Society of Plastics Engineers     

Polymerization compounding composites of nylon‐6,6/short glass fiber

Nylon‐6,6 was grafted onto the surface of short glass fibers through the sequential reaction of adipoyl chloride and hexamethylenediamine onto the fiber surface. Grafted and unsized short glass fibers (USGF) were used to prepare composites with nylon‐6,6 via melt blending. The glass fibers were found to act as nucleating agents for the nylon‐6,6 matrix. Grafted glass fiber composites have higher crystallization temperatures than USGF composites, indicating that grafted nylon‐6,6 molecules further increase crystallization rate of composites. Grafted glass fiber composites were also found to have higher tensile strength, tensile modulus, dynamic storage modulus, and melt viscosity than USGF composites. Property enhancement is attributed to improved wetting and interactions between the nylon‐6,6 matrix and the modified surface of glass fibers, which is supported by scanning electron microscopy (SEM) analysis. The glass transition (tan δ) temperatures extracted from dynamic mechanical analysis (DMA) are found to be unchanged for USGF, while in the case of grafted glass fiber, tan δ increases with increasing glass fiber contents. Moreover, the peak values (i.e., intensity) of tan δ are slightly lower for grafted glass fiber composites than for USGF composites, further indicating improved interactions between the grafted glass fibers and nylon‐6,6 matrix. The Halpin‐Tsai and modified Kelly‐Tyson models were used to predict the tensile modulus and tensile strength, respectively.     

Studies on surface energetics of glass fabrics in an unsaturated polyester matrix system: Effect of sizing treatment on glass fabrics

The surfaces of glass fibers were sized by polyvinyl alcohol (PVA), polyester, and epoxy resin types in order to improve the mechanical interfacial properties of fibers in the unsaturated polyester matrix. The surface energetics of the glass fibers sized were investigated in terms of contact angle measurements using the wicking method based on the Washburn equation, with deionized water and diiodomethane as the wetting liquids. In addition, the mechanical behaviors of the composites were studied in the context of the interlaminar shear strength (ILSS), critical stress intensity factor (K_IC), and flexural measurements. Different evolutions of the London dispersive and specific (or polar) components of the surface free energy of glass fibers were observed after different sizing treatments. The experimental result of the total surface free energies calculated from the sum of their two components showed the highest value in the epoxy‐sized glass fibers. From the measurements of mechanical properties of composites, it was observed that the sizing treatment on fibers could improve the fiber–matrix interfacial adhesion, resulting in improved final mechanical behaviors, a result of the effect of the enhanced total surface free energy of glass fibers in a composite system. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1439–1445, 2001     

Essential work of fracture (EWF) analysis for short glass fiber reinforced and rubber toughened nylon‐6

The effect of fiber content on the fracture toughness of short glass fiber reinforced and rubber toughened nylon‐6 has been investigated using the essential work of fracture (EWF) analysis under both quasi‐static and impact rates of loading. Under quasi‐static loading rate, matrix plastic deformation played a major role. Addition of 10 wt% of short glass fibers into a rubber toughened nylon‐6 matrix improved the fracture toughness substantially. This is due to the synergistic effect that comes from matrix yielding and fiber related energy absorption such as fiber debonding, fiber pull‐out and fiber fracture. With further increasing the glass fiber content, up to 20 and 30 wt%, even though plastic deformation could still take place on the fracture surfaces, the depth of the fracture process zones was much smaller when compared with the system with 10 wt% of glass fibers. The reduction in fracture process zone caused the reduction in fracture toughness. Under impact loading rate, the unreinforced blend still fractured in a ductile manner with gross yielding in the inner fracture process zone and the outer plastic zone. The unrein‐forced blend therefore possesseed higher fracture toughness. For the fiber reinforced blends, the matrix fractured in brittle manner and so fracture toughness of the reinforced blends decreased dramatically. The impact fracture toughness increased slightly after incorporation of a higher weight percentage of glass fibers.     

Studies on mechanical characterization and water resistance of glass fiber/thermoplastic polymer bionanocomposites

This experimental work is aimed at studying the performance of rice husk flour/glass fiber reinforced high density polyethylene hybrid nanocomposites. To meet this objective, the nanoclay was compounded with high density polyethylene (HDPE), rice husk flour (RF), glass fiber, and coupling agent in an internal mixer; then, the samples were fabricated by injection molding. The concentration was varied from 0 to 6 per hundred compounds for nanoclay and from 0 to 15% for glass fiber, individually. The amount of coupling agent was fixed at 2% for all formulations. The morphology, water absorption, thickness swelling, and mechanical properties of nanocomposites were evaluated as a function of nanoclay and glass fiber contents. The results indicated that both modulus and strength were improved when glass fibers were added to the composites system but impact strength and moisture absorption further decreased with the increase of glass fiber content. The morphology of the nanocomposites has been examined by using X‐ray diffraction. The morphological findings revealed that the nanocomposites formed were intercalated. The mechanical analysis showed that the biggest improvement of the tensile and flexural modulus and strengths can be achieved for the nanoclay loading at 4 per hundred compounds. However, further increasing of the loading of nanoclay resulted in a decrease of impact strength. Finally, it was found that addition of nanoclay reduced the water absorption and thickness swelling of the composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Tribological behavior of short‐fiber‐reinforced polyimide composites under dry‐sliding and water‐lubricated conditions

Polyimide composites reinforced with short‐cut fibers such as carbon, glass, and quartz fibers were fabricated by the polymerization of monomer reactants process. The mechanical properties of the composites with different fiber contents were evaluated. The friction and wear properties of the polyimide and its composites were investigated under dry‐sliding and water‐lubricated conditions. The results indicated that the short‐carbon‐fiber‐reinforced polyimide composites had better tensile and flexural strengths and improved tribological properties in comparison with glass‐fiber‐ and quartz‐fiber‐reinforced polyimide composites. The incorporation of short carbon fibers into the polyimide contributed to decreases in the friction coefficient and wear rate under both dry and water‐lubricated conditions and especially under water lubrication because of the boundary lubrication effect of water. The polyimide and its composites were characterized by plastic deformation, microcracking, and spalling under both dry and water‐lubricated conditions, which were significantly abated under the water‐lubricated condition. The glass and quartz fibers were easily abraded and broken; the broken fibers transferred to the mating metal surface and increased the surface roughness of mating stainless steel, which led to the wear rate increasing for the glass‐fiber‐ and quartz‐fiber‐reinforced polyimide composites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

The adhesion of elastomer-coated glass fibers to an epoxy matrix. Part I: The effect of the surface treatments on the tensile strength of the glass fibers

E-glass fibers were subjected to various surface treatments to study the interfacial adhesion with an epoxy matrix by means of the fragmentation test. The glass fibers considered were both untreated and treated with γ-aminopropyltriethoxysilane (γ-APS). In addition, glass fibers were coated with a thin layer of a crosslinkable elastomer including (or not) a silane coupling agent. To evaluate the effect of the coating process, the glass fibers were also passed through the pure solvent (washed fibers). The tensile strength of glass fibers at short length (near the critical length, l_c, for the fragmentation test) cannot be measured directly, thus, extrapolation techniques were used. The Weibull statistics technique was applied and accurately described the tensile strength data of the tensile strength at various fiber gauge lengths for the different surface treatments. Nevertheless, the Weibull parameter, m, changes with the gauge length, and therefore, the extrapolation value at l_c depends on the method. The tensile strength of the silane-treated glass fibers is higher than for the untreated fibers, in agreement with reported data. This effect could be attributed to the protection against water provided by the silane sizing. The coating process does not induce any damage of the glass fibers since the washed fibers display mechanical properties close to those obtained for the untreated fibers. An additional effect is observed for the elastomer-coated fibers which present the highest tensile strength. This effect could be attributed to an improved protection and/or elimination of the weakest filaments in the glass strand during the treatment.     

Evaluation of surface treatments for glass fibers in composite materials

The thermomechanical stability of a number of organosilane surface treatments for glass fibers was evaluated for use in a fiber reinforced epoxy resin. All of the silane coatings were found to improve the tensile strength of E-glass filaments, particularly at large gauge lengths. A phenylamino silane and an amino silane were particularly effective in this regard. The fiber/matrix interface was evaluated as a function of temperature and after exposure to boiling water using a single-fiber composite test. All silane coatings transmitted a higher interfacial shear stress than obtained in composites with no coatings, and in all cases the shear stress transmission was considerably higher than would be expected from the yield properties of the resin. Measurements of the glass transition temperature of the epoxy resin, as well as Fourier-Transform Infra-Red analysis, indicated modification of resin properties in a zone around the glass fibers. Each of the silane coatings provided more stable thermomechanical properties than those obtained with uncoated glass, at least until the silanes were irreversibly degraded by boiling water. A phenylamino silane provided the most thermally stable properties. Finally, unidirectional E-glass fiber reinforced laminae were fabricated and the measured values of longitudinal strength were compared favorably to theoretical predictions.     

MICROSTRENGTH OF GLASS *

It is shown experimentally that if a minute area of a large piece of glass is tested for tensile strength, the stress supported is comparable with that of fine glass fibers and is some fifty or one hundred times the “strength” of the large piece tested in any of the ordinary ways. The experiment consists in pressing a small steel ball upon the glass surface, and so stressing, in tension, a minute annular zone around the circle of contact.     

Effects of chemical surface pretreatment on tensile properties of a single glass fiber and the glass fiber reinforced epoxy composite

The aim of this article is to determine the effect of surface pretreatments, prior to the silanization, on the structure and tensile properties of the glass fibers and their epoxy composites. Commercial glass fibers were washed with acetone to remove the soluble portion of sizing, calcinated for the removal of organic matter, activated for surface silanol regeneration, and silanizated with glycidoxypropyltrimethoxysilane (GPS). Tensile test was carried out. The morphology of pretreated glass fibers and the fracture surfaces of the epoxy composites were observed with a scanning electron microscope (SEM). The results revealed that both apparent modulus and strength of a single glass fiber and the glass fiber/epoxy resin composites strongly depend on the fiber surface pretreatments. The acetone treatment did not change appreciably the composition and tensile properties of glass fibers, but there was a weak interface between fibers and matrix. In calcinated and acid activated fibers, the two competitive effects was observed: (1) degradation of the fibers themselves and (2) improved interfacial adhesion between the glass fibers and the epoxy matrix, once the samples was silanizated. The ATR‐FTIR results show that the surface content of Si OH increases as reflected by the increasing of the Si O band, resulting in an interaction between silane coupling agent and glass fiber. POLYM. COMPOS., 91–100, 2016. © 2014 Society of Plastics Engineers     

Electrospun nano‐scaled glass fiber reinforcement of bis‐GMA/TEGDMA dental composites

The objective of this study was to investigate the electrospun nano‐scaled glass fiber reinforcement of 2,2′‐bis[4‐(methacryloxypropoxy)‐phenyl]‐propane/triethylene glycol dimethacrylate (Bis‐GMA/TEGDMA) dental composites. The hypothesis was that incorporation of the surface‐silanized electrospun nano‐scaled glass fibers into Bis‐GMA/TEGDMA dental composites would result in substantial improvement on mechanical properties. To test the hypothesis, photo‐cured Bis‐GMA/TEGDMA dental composites filled with various mass fractions of surface‐silanized electrospun nano‐scaled glass fibers were systematically fabricated; and their mechanical properties were then evaluated. The results indicated that small mass fraction substitutions (1, 2.5, 5, and 7.5%) of conventional dental filler with the surface‐silanized electrospun nano‐scaled glass fibers, significantly improved the flexural strength, elastic modulus, and work of fracture values of 70% (mass fraction) filled composites, by as much as 44%, 29%, and 66%, respectively. The mechanical properties of the composites could be further improved by optimizing the chemical compositions and the surface treatment methods of the fibers. We envision that the electrospun nano‐scaled glass fibers could be utilized to develop the next generation dental composites, which would be particularly useful for large posterior restorations. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Development of a poly(N‐vinyl‐2‐pyrrolidone)/poly (ethylene glycol) hydrogel membrane reinforced with methyl methacrylate‐grafted polypropylene fibers for possible use as wound dressing

A hydrogel is a polymeric material that exhibits the ability to swell in water and retains a significant fraction of water within its structure, but does not dissolve in water. One of the major problems in the application of these materials is their relatively poor mechanical strength, attributed to the high degree of hydration of the gel. This work was directed to the study of the interactions between hydrophobic and hydrophilized fibers, with the objective of optimization of the mechanical properties of poly(N‐vinyl‐2‐pyrrolidone) membranes. The membranes were prepared by electron‐beam irradiation of an aqueous polymer solution. A nonwoven cloth made of polypropylene matted fiber, grafted with methyl methacrylate, was employed as a reinforcement. The changes in the main properties of the membranes, such as the gel content, swelling characteristics, cytotoxicity, and mechanical behavior, were investigated. The results showed an increase of 800% in tensile strength, without changes in the swelling and cytotoxicity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 662–666, 2002     

An efficient way to improve the mechanical properties of polypropylene/short glass fiber composites

Enhancement of tensile strength, impact strength, and flexural strength of polypropylene/short glass fiber composites by treating the glass fibers with coupling agent, mixing with maleated polypropylene (MPP) for compatibilization and adhesion, and with nucleating agent for improvement of polypropylene crystallization was studied. The results showed that both the silane coupling agent and MPP enhance tensile strength, impact strength, and flexural strength. In the absence of MPP, the effect of silane coupling agent on the mechanical properties of the composites decreases in the following order: alkyl trimethoxy silane (WD‐10) > γ‐methacryloxypropyl trimethoxysilane (WD‐70) > N‐(β‐aminoethyl)‐γ‐aminopropyl trimethoxysilane (WD‐52), whereas in the presence of MPP, the order changes as follows: WD‐70 > WD‐10 > WD‐52. When the glass fibers were treated with WD‐52, 4,4‐diamino‐diphenylmethane bismaleimide (BMI) can further enhance the mechanical properties of the composite. The three kinds of strengths increase with MPP amount to maximum values at 5% MPP. As a nucleating agent, adipic acid is better than disodium phthalate in improving the mechanical properties, except for the notched impact strength. Wide‐angle X‐ray diffraction showed that the adipic acid is an α‐type nucleating agent, whereas disodium phthalate is a β‐type nucleating agent. Blending with styrene–butadiene rubber can somewhat improve the notched impact strength of the composites, but severely lowers the tensile strength and bending strength. Scanning electron micrographs of the broken surface of the composite showed greater interfacial adhesion between the glass fibers and polypropylene in the modified composite than that without modification. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1414–1420, 2005     

Composites based on poly(ethylene terephthalate) fibers with polyaniline. I. Effect of the aniline monomer in the morphology of the PET substrate

Heat‐treated PET fibers of different draw ratios (0X, 2.6X) were submitted to different conditions (time and temperature) of chemical treatments in the presence of aniline and aniline plus benzoic acid in order to verify the influence of these chemicals on their structure. Both crystalline structures of these fibers behaved similarly in some aspects to these applied chemical treatments, but differently in others. The major differences would be related to their original structures (control ones) due to the existence or not of previous orientation. The drawn fiber presented a more stable and more complex structure than that of the undrawn one. The crystalline structure of the drawn fibers changed from a two‐form and more heterogeneous crystalline structure to a more homogeneous one, which is constituted of smaller and/or less perfect crystallites. The use of benzoic acid in combination with aniline seems to be more effective to plasticize both fibers, especially at higher temperatures and longer times of treatment. At these conditions, the fibers presented swelling and a more effective reduction of their glass transition temperatures (T_g). The swelling and the increase in the chain flexibility might be responsible for the formation of the more homogeneous crystalline structure of the drawn fibers. The swelling promoted not only disorientation of the amorphous regions, but disorientation of their crystalline regions as well. This phenomenon seems to be responsible for the observed decrease in the intensity of the X‐ray diffractograms within a given treatment for both fibers. Sonic modulus analysis performed for the drawn fibers confirmed such data. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2126–2138, 2000